As a decorator I have used many types of face masks in my career and have found some face masks were good and others much less so.  The cheap disposable face masks are in my opinion ineffective, as I find they fail to stop the dust particles penetrating the inner mask area and therefore my lungs.   The problem with these types of masks is their inability to create an air-proof seal between them and my face, particularly around the nose and under the chin.  That said it is always better to wear an ill-fitting face mask than none at all, as any form of protection from dust and fibre particles is better than none.

Wear a face mask that fits the contours of your face and generates an effective and continuous seal against the skin.    Face masks can only protect you from harmful particles, if they fit.       The seal around the upper nose is the most difficult to form, and for those of us who wear glasses a broken seal here often results in steamed up on the lenses of spectacles which can be both annoying and dangerous.  

Some Face masks not only give you effective protection against dust particles, but they can also protect you from vapours such as paint fumes, however to achieve this level of protection, it is essential vapour specific mask be purchased and then correctly fitted.  The mask I personally use that matches these requirements is the 3M Maintenance Free Respirator 4251 - Dust Mask, you can read more about this mask on my product review post, 3M 4251 Respirator.